I. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method and apparatus for controlling the transmit power of a high rate code division multiple access link.
II. Description of the Related Art
FIG. 1 is a schematic diagram of a cellular telephone system configured in accordance with the use of the IS-95 over-the-air interface standard. The IS-95 standard, and its derivatives such as IS-95-A, IS-99, IS-657 and ANSI JSTD-008 etc. (referred to herein collectively as the IS-95 standard), define an interface for implementing a digital cellular telephone system using code division multiple access (CDMA) signal processing techniques. Also, a cellular telephone system configured substantially in accordance with the use of IS-95 is described in U.S. Pat. No. 5,103,459 entitled "System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System" assigned to the assignee of the present invention and incorporated herein by reference.
As is typical for most cellular telephone systems, IS-95 allows mobile telephone service to be provided using a set of base stations 12 coupled to the public switched telephone network (PSTN) 18 by a base station controller (BSC) 14 and a mobile switching center (MSC) 16. During a telephone call, a subscriber unit 10 (typically a cellular telephone) interfaces with one or more base stations 12 using CDMA modulated radio frequency (RF) signals. Throughout this application, the term subscriber unit refers generically to a communications device remotely located from a base station 12, and includes mobile units, hand units, and all other similar communication devices. The RF signal transmitted from the base station 12 to the subscriber unit 10 is referred to as the forward link, and the RF signal transmitted from the subscriber unit 10 to the base station 12 is referred to as the reverse link.
Unlike most other cellular telephone systems, IS-95 communication systems are capable of performing soft handoff when a subscriber unit 10 transitions from the coverage area of a first base station 12 to the coverage area of a second base station 12. In soft handoff, the subscriber unit 10 establishes a communication link with the second base station 12 before terminating the communication link with the first base station. Thus, soft handoff requires simultaneously interfacing with both the first and second base stations 12, which is the condition of subscriber unit 10b as shown in FIG. 1. The preferred embodiments for performing soft handoff are described in U.S. Pat. No. 5,267,261 entitled "Mobile Station Assisted Soft Handoff in a CDMA Cellular Communications System" assigned to the assignee of the present invention and incorporated herein by reference. Soft handoff can be contrasted with hard handoff in which the link with the first base station is terminated before the link with the second base station is established.
Soft handoff is generally necessary in a CDMA based cellular telephone system because adjacent base stations transmit in the same RF band, and the interference between the two forward link signals at the border of the coverage areas fluctuates rapidly and unpredictably. The fading caused by this interference results in a poor received signal to noise ratio at the subscriber unit 10, which further results in either a higher required transmit power from the base station 12, or a higher error rate, or a combination thereof. During soft handoff, both the first and second base stations 12 transmit copies of the user data directed to subscriber unit 10 to provide signal source diversity. Thus, if one base station 12 fades relative to the other base station 12, the subscriber unit 10 still receives one signal properly. Also, the two forward link signals can be combined at the subscriber unit 10, which can result in proper reception even when neither forward link signal alone was received at a sufficient level.
It should be noted that soft handoff can also involve three or more base stations 12, all transmitting copies of the data directed to subscriber unit 10. In general, the signal diversity provided by soft handoff makes it more robust than hard handoff, in that the call is less likely to be dropped.
Although soft handoff has the advantage of being more robust than hard handoff, it has the disadvantage of requiring two or more transmissions of the same user data. Within an IS-95 compliant or other type of CDMA cellular telephone system, these multiple transmissions may increase or decrease overall capacity of the cellular telephone system. Whether there is an increase or decrease depends of the fading condition for the subscriber unit 10.
More recently, however, it has become desirable to provide higher transmission rate communication services in conjunction with the mobile telephone service already provided by an IS-95 cellular telephone system. Examples of such higher rate links are described in copending U.S. patent applications Ser. No. 08/656,649 filed May 31, 1996 entitled "Method and Apparatus for Providing Rate Scheduled Data in a Spread Spectrum Communication System and Ser. No. 08/784,280 entitled "High Data Rate Supplemental Channel for CDMA Telecommunications System" filed Jan. 15, 1997 both assigned to the assignee of the present invention and incorporated herein by reference (high data rate patent applications).
These higher rate communications are transmitted with substantially greater power than the typical voice based communications, which substantially increases the negative impact of generating multiple transmissions as during a soft handoff. Since it is nonetheless desirable to allow a subscriber unit 10 conducting a high speed communication to transition between the coverage areas of different base stations 12, an alternative method and apparatus for performing handoff is required.